1. Field of the Invention
The present invention relates to an apparatus for displaying video images and, more particularly, to a cathode ray tube apparatus having a deflection yoke.
2. Description of the Prior Art
Typically, a cathode ray tube (CRT) apparatus produces electron beams which may be deflected electromagnetically so as to enable the electron beams to properly scan a designated area on a screen. An example of such CRT apparatus is illustrated in FIG. 1.
As shown in FIG. 1, a CRT apparatus 10 generally includes an envelope portion 12, an electron gun or guns 14, a deflection yoke (DY) 16, a shadow mask or aperture grill 20, and a screen or panel 18. The envelope portion 12, which may be fabricated from glass, ceramic or metal like material, includes a neck portion 13 wherein the electron gun(s) 14 are mounted. Each such electron gun 14 is adapted to produce a beam of electrons.
In a monochrome CRT apparatus, there is normally one electron gun, whereas in a color CRT apparatus there are normally three electron guns. In the following discussion, the CRT apparatus 10 will be described as a color CRT apparatus, although such CRT apparatus may be a monochrome type CRT apparatus.
The DY 16 may include a plurality of sections or pairs of shaped coil members, such as two shaped coil members 16a and 16b. The DY 16 may further include a vertical coil 16c which may be wound around a ferrite core and a horizontal coil 16d. The members 16a and 16b are arranged around the neck portion 13 in a predetermined manner. Such arrangement results in gaps or spaces between various positions on an inner surface 72 of the DY 16 and corresponding positions on an outer surface 70 of the envelope 12. More specifically, a gap 80 may exist between the inner surface 72 of a rear portion 24 of the DY 16 and a corresponding portion 50 of the outer surface 70, a gap 82 may exist between the inner surface 72 of a middle portion 23 of the DY 16 and a corresponding portion 52 of the outer surface 70, and a gap 84 may exist between the inner surface 72 of a front portion 22 of the DY 16 and a corresponding portion 54 of the outer surface 70.
The DY 16 and, in particular, the inner surface 72 thereof, is fabricated and arranged on the envelope 12 such that the values of the gaps 80, 82 and 84 increase from the rear portion to the front portion. That is, the value of the gap 80 has the smallest value, the value of the gap 84 has the largest value, and that of the gap 82 has a value between that of gaps 80 and 84. Further, gap 80 has a relatively small value and may have a value of zero, whereupon the inner surface 72 of the rear portion 24 of the DY 16 contacts the portion 50 of the outer surface 70. Furthermore, the other gaps 82 and 84 have relatively small values. The values of gaps 82 and 84 typically enable the DY 16 to be tilted by approximately 2.degree. around gap 80 (or the portion relating thereto) which effectively functions as a pivot point. The gap 84 may be utilized for tilt adjustment so as to compensate for misalignment which may be due to tolerance errors or the like.
The DY 16, in response to a signal, such as a pulsating current signal, is adapted to produce an electromagnetic field having predetermined characteristics or shape. As a result of such electromagnetic field, the three beams from the three electron guns 14, which respectively correspond to different colors, such as red, green and blue (RGB), are deflected in a predetermined manner.
The screen 18 may have a spherical shape. The shadow mask or aperture grill 20 is located relatively close to the screen 18. Such shadow mask or aperture grill may include a plurality of relatively small apertures, which may have a round or elongated slit shape, each respectively associated with a group of red, green and blue light-emitting elements (not shown) of the screen 18. The shadow mask or aperture grill 20 is adapted to enable the electron beams from the three electron guns 14 to properly strike the appropriate light-emitting elements of the screen 18.
Therefore, electron beams produced by the electron guns 14 are deflected due to the electromagnetic field created by the DY 16 so as to pass through the aperture grill 20 and strike the appropriate light-emitting elements of the screen 18. As a result, a color image is displayed on the screen 18.
The video image provided on the screen 18 may be distorted by so-called left/right or East/West (EW) pin-cushion distortion of the geometric raster and other types of geometric and misconvergence distortions. Examples of a displayed video image having such distortion are illustrated in FIGS. 2a and 2b. That is, FIGS. 2a and 2b illustrate a video image displayed on the screen 18 having an EW pin-cushion shaped distortion of a rectangular raster and an EW barrel shaped distortion, respectively.
In an attempt to correct for EW pin-cushion distortion, the magnetic field created by the vertical deflection coil 16c of the DY 16 may be shaped so as to have a pin-cushion shape at the front portion 22 of the DY 16 and a barrel shape at the rear portion 24 of the DY 16. An example of such vertical magnetic field having pin-cushion shaped field or flux lines 46 and barrel shaped field or flux lines 48 are illustrated in FIGS. 3 and 4, respectively. A deflection yoke 116, as illustrated in FIG. 5, may be utilized in an attempt to obtain such magnetic field. Such distortion and magnetic fields are described in an article entitled "Designing Self-Converging CRT Deflection Yokes", by Basab B Dasgupta, Information Display, 1/92, pp 15-19, which is hereby incorporated by reference.
As shown in FIG. 5, a member such as a so-called mold core device 135 is coupled to a rear portion 124 of the DY 116. The mold core device 135 is typically formed from a magnetic material having relatively high permeability by use of a molding process. Such mold core device produces a magnetic field having barrel-shaped field lines.
As further shown in FIG. 5, a member 130 known as a cross-arm (i.e., cold rolled silicon steel arm) is coupled to a front portion 122 of the DY 116. Such cross-arm member 130, as shown in FIG. 6, includes a plurality of members, such as four members 131-134, which have respective effective magnetic poles associated therewith and which are arranged in a predetermined manner. For example, members 131 and 133 may each be a north (N) magnetic pole, and members 132 and 134 may each be a south (S) magnetic pole. As a result of such arrangement, the cross-arm 130 is adapted to reshape the vertical magnetic field so as to have field lines 136 having a pin-cushion shape.
Alternatively, a pin-cushion shaped vertical coil field at the front and a barrel shaped field at the rear may be obtained by utilizing a biased winding in which the angular width of the coil is relatively narrow at the front and relatively wide at the rear. Examples of unbiased and biased coils are illustrated in FIGS. 12A-12D. That is, FIG. 12A illustrates an unbiased coil, FIG. 12B illustrates a severely biased coil, FIG. 12C illustrates a slightly biased coil, and FIG. 12D illustrates an unbiased coil.
Although the above-described DY 116 may provide a magnetic field with field lines having a barrel shape at the rear portion 124 and a pin-cushion shape at the front portion 122, such configuration may not always cause a video image to be displayed on a screen of a CRT apparatus in which distortion such as EW pin-cushion distortion of the geometric raster and other geometric and misconvergence distortions are eliminated or reduced to an acceptable level. As an example, consider a CRT apparatus having a panel with a cylindrical shape and which is relatively flat, such as model no. SD169-20V 100.degree. CRT manufactured by the Sony Corporation a diagram of which is illustrated in FIG. 7.
As shown in FIG. 7, a CRT apparatus 200 generally includes an envelope 212, an electron gun 214, a deflection yoke (DY) 216, an aperture grill 220, and a screen or panel 218.
The envelope 212, which may be fabricated from glass, has a funnel portion 213. Such funnel portion may be a 100.degree. funnel. The electron gun 214, which may be a Trinitron electron gun, is arranged within the funnel portion 213. The DY 216, having coil members 216a and 216b, having a vertical coil 216c which may be wound around a ferrite core and a horizontal coil 216d, is arranged around the funnel portion 213 as shown in FIG. 7 and is adapted to produce an electromagnetic field so as to deflect the electron beams from the electron gun 214 in a predetermined manner. Further, as shown in FIG. 7, the DY 216 is arranged or coupled to the envelope 212 so as to have the gaps or spaces 80, 82 and 84 between an inner surface 90 of the DY 216 and an outer surface 92 of the envelope 212 from a rear portion 224 to a front portion 22 in a manner similar to that previously described with reference to the DY 16 of FIG. 1.
The screen 218 and aperture grill 220 function in a manner similar to that of the screen 18 and aperture grill 20 of the CRT apparatus 10 of FIG. 1 and, as such, will not be further described herein. However, unlike the screen 18, the screen 218 has substantially a cylindrical shape, such as that shown in FIG. 8. Furthermore, the panel 218 has a relatively flat surface, that is, the radius R of the panel 218 has a relatively large value. For example, the radius R of the panel 218 may have a value of approximately 1.5 to 2.0 times that of similar size panels of similar type CRT apparatuses, such as a 20V CRT manufactured by either the RCA Corporation or the Zenith Corporation. Such cylindrical and relatively flat shape of the panel 218 may provide a more acceptable viewing surface to an observer as compared to other less flat and spherical shape panels, such as that of CRT apparatus 10.
Even if the deflection yoke 216 is configured in a manner similar to that of the deflection yoke 116 of FIG. 5, the CRT apparatus 200 may not provide a video image on the screen 218 which is either free of distortions, such as EW pin-cushion distortion of the geometric raster and other geometric and misconvergence distortions, or in which such distortions are reduced to an acceptable level. The difficulty in obtaining such distortion free (or reduced distortion) video image is due, at least in part, to the cylindrical and relatively flat shape of the panel 218.
Thus, although the cylindrical and relatively flat shape of the panel 218 may provide a more acceptable viewing surface to an observer as compared to other less flat and spherical shape panels, such cylindrical and relatively flat shaped panel 218 may increase or aggravate the distortion of the displayed video image.
Therefore, the prior art has failed to provide a CRT apparatus, such as a CRT apparatus having a cylindrical and relatively flat panel, having a deflection yoke which produces a magnetic field such that a video image may be displayed on a screen of the apparatus without EW pin-cushion distortion of the geometric raster and other geometric and misconvergence distortions or with a minimum or relatively low amount of such distortion.